I think, because it's "cute" and that's the main thing with anime now-a-days also the cat guy and I guess some people like the clothes. Oh! and another trend is the "random" humor. *shrugs* I don't really like it.

i like shugo chara cause well......im not sure i got really interested when i got the first manga.........and then that's it im not really a big fan of ikuto (the cat guy) .....i jus luv the twists in the story and soo on..........
and there are lots of couples..........

Because it's a kids show. The art style is cute. MOSTLY girls watch it. Most people that like it are between 11-14ish. Which is a very large group of fangirls. That's why it gets spread everywhere with "OMG IKUTO IS SO KAWAII. XDXDXD"